[PATCH] D47041: [ValueTracking] Teach computeKnownBits that the result of an absolute value pattern that uses nsw flag is always positive.

[Craig Topper via Phabricator via llvm-commits]

craig.topper updated this revision to Diff 148247.
craig.topper added a comment.

Rebase to remvoe the InstCombine changes that were split off to another patch and have now been committed


https://reviews.llvm.org/D47041

Files:
  lib/Analysis/ValueTracking.cpp
  test/Transforms/InstCombine/abs-1.ll
  test/Transforms/InstCombine/abs_abs.ll


Index: test/Transforms/InstCombine/abs_abs.ll
===================================================================
--- test/Transforms/InstCombine/abs_abs.ll
+++ test/Transforms/InstCombine/abs_abs.ll
@@ -933,8 +933,8 @@
 ; CHECK-LABEL: @nabs_abs_x09(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[X]]
-; CHECK-NEXT:    [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
-; CHECK-NEXT:    ret i32 [[COND1]]
+; CHECK-NEXT:    [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT:    ret i32 [[SUB16]]
 ;
   %cmp = icmp sgt i32 %x, -1
   %sub = sub nsw i32 0, %x
@@ -949,8 +949,8 @@
 ; CHECK-LABEL: @nabs_abs_x10(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[X]]
-; CHECK-NEXT:    [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
-; CHECK-NEXT:    ret i32 [[COND1]]
+; CHECK-NEXT:    [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT:    ret i32 [[SUB16]]
 ;
   %cmp = icmp sgt i32 %x, 0
   %sub = sub nsw i32 0, %x
@@ -965,8 +965,8 @@
 ; CHECK-LABEL: @nabs_abs_x11(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[X]]
-; CHECK-NEXT:    [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
-; CHECK-NEXT:    ret i32 [[COND1]]
+; CHECK-NEXT:    [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT:    ret i32 [[SUB16]]
 ;
   %cmp = icmp slt i32 %x, 0
   %sub = sub nsw i32 0, %x
@@ -981,8 +981,8 @@
 ; CHECK-LABEL: @nabs_abs_x12(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[X]]
-; CHECK-NEXT:    [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
-; CHECK-NEXT:    ret i32 [[COND1]]
+; CHECK-NEXT:    [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT:    ret i32 [[SUB16]]
 ;
   %cmp = icmp slt i32 %x, 1
   %sub = sub nsw i32 0, %x
Index: test/Transforms/InstCombine/abs-1.ll
===================================================================
--- test/Transforms/InstCombine/abs-1.ll
+++ test/Transforms/InstCombine/abs-1.ll
@@ -293,3 +293,14 @@
   %r = sub <2 x i8> zeroinitializer, %s
   ret <2 x i8> %r
 }
+
+define i1 @abs_must_be_positive(i32 %x) {
+; CHECK-LABEL: @abs_must_be_positive(
+; CHECK-NEXT:    ret i1 true
+;
+  %negx = sub nsw i32 0, %x
+  %c = icmp sge i32 %x, 0
+  %sel = select i1 %c, i32 %x, i32 %negx
+  %c2 = icmp sge i32 %sel, 0
+  ret i1 %c2
+}
Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -1078,6 +1078,12 @@
       // leading zero bits.
       MaxHighZeros =
           std::max(Known.countMinLeadingZeros(), Known2.countMinLeadingZeros());
+    } else if (SPF == SPF_ABS) {
+      // RHS from matchSelectPattern returns the negation part of abs pattern.
+      // If the negate has an NSW flag we can assume the sign bit of the result
+      // will be 0 because that makes abs(INT_MIN) undefined.
+      if (cast<Instruction>(RHS)->hasNoSignedWrap())
+        MaxHighZeros = 1;
     }
 
     // Only known if known in both the LHS and RHS.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D47041.148247.patch
Type: text/x-patch
Size: 3258 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20180523/46ee7318/attachment-0001.bin>